As part of strategic emerging industries in China, high-strength fibers including carbon fibers, aramid fibers, polyethylene and fiberglass and the composite material products thereof have the advantages of light weight, high strength, corrosion resistance and unique concealment performance etc. Composite materials, which are widely applied in fields including wind energy, aeronautics and astronautics, automobiles, railway communication, buildings, weapons, armors, ships, chemical engineering and sports etc., have been an important fiercely-competitive industry that is developed by countries all over the world as a priority. Composite materials are basic key materials in sophisticated industries including aeronautics and astronautics etc. For example, composite material technology are the most critical technology in the competition between Boeing and Airbus as well as one of the major bottlenecks of civil aircraft projects in China. The composite materials used in Boeing 787 already account for more than 50% of the total mass of the plane. Shells of stealth fighters are basically made of microwave absorbing composite materials. In the meanwhile, composite materials are one of the basic factors for stealth of planes and naval vessels. Although having many excellent performances, the following disadvantages need to be overcome to expand the application of composite materials:
1. Easy Interlaminar Cracking
Most existing fiber composite materials are produced by superimposing fiber sheets including fiber cloth and prepregs etc. to a certain thickness and cure the fiber sheets by resin substrates. Thanks to the ultra-high strength fibers on the surfaces in 2 dimensions of the sheets, strength of the sheets are several times stronger than that of steel and may reach above 3000 MPa. However, there are resin plastic substrates among the sheets, and the interlaminar strength are extremely low at just 100 MPa. The difference between the fiber strength in the layers and the plastic strength among the layers is as much as more than 30 times. Therefore, easy interlaminar cracking is an intrinsic disadvantage of fiber composite materials. Because of the weak interlaminar strength, as well as the relatively low impact strength and compressive strength, interlaminar cracking is the main failure of composite materials, especially when impacted and compressed to cause fatigue.
Methods including interlaminar stitching, three-dimensional spinning and three-dimensional weaving etc. may be applied in order to improve the interlaminar strength of composite materials. Although some achievements have been made in the research and development, these technologies have complicated processes together with very high cost and limited use. Nevertheless, broadly-applied multi-axial warp knitted composite materials fail to obtain three-dimensional structures due to the thickness limitation. So, interlaminar cracking is the major disadvantage that harasses the performance of composite materials. Therefore, it's been a problem in the world to enhance the interlaminar strength of composite materials at low costs.
2. Low Lamination Efficiency and High Labor Costs
Usually, if long staples are required to be used as structural materials, fiber sheets are manufactured by yarns and composite material plates or products are produced by superimposing layers of fiber sheets to a certain thickness. Processes of production of yarns, fabrics, plies/composites are necessary in the application of long staples as materials. However, only the process of fabricating yarns into fabrics can be realized efficiently by spinning techniques in the whole production process of fiber composite material products. Since fiber sheets can be hardly operated automatically and mechanically, expensive automatic fiber orientation devices can be applied only in sophisticated industries that require very high lamination accuracy of fiber sheets, such as aircraft manufacturing. Therefore, fiber sheets are mostly laminated into plates and products manually in the industry of composite materials, which is low in production efficiency and high in labor cost, wherein the low manual lamination efficiency has always been the main bottleneck of the production process of composite materials.
3. Expensive High-Strength Fibers Including Carbon Fibers, Aramid Fibers and High-Modulus Polyethylene Etc.
The low interlaminar strength, the low lamination efficiency and the high labor costs of lamination processes of fiber composite materials result in limited application of composite materials and limited demands of high-strength fibers including carbon fibers, aramid fibers and high-modulus polyethylene etc. that are mainly used in high-end products in the market. Along with the technical monopoly of developed countries on carbon fibers, aramid fibers and high-modulus polyethylene, these high-strength fibers are naturally very expensive. The good news is that production problems of carbon fibers and high-modulus polyethylene have been solved in China in recent years to realize localization, and aramid fibers will be produced at home soon.
If the interlaminar strength of composite materials are improved and composite materials can be laminated automatically at low costs, the application demands of composite materials will increase inevitably, the yields of carbon fibers, aramid fibers and high-modulus polyethylene will be also increased greatly and their manufacturing costs are expected to decrease.